1. Field of the Invention
The present invention generally relates to an oxide superconductor current lead and to a method of manufacturing such an oxide superconductor current lead. More particularly, the present invention relates to an oxide superconductor current lead to be used when supplying a large current (flow) to a superconducting coil cooled by liquid helium, and to a suitable method for manufacturing such an oxide superconductor current lead.
2. Description of the Related Art
Generally, a current lead is used as means for supplying a large current to a superconducting coil placed in liquid helium. Further, current leads made of metallic copper are the mainstream of conventional current leads.
As shown in FIG. 3, it is quite common that in conventional superconducting equipment, first, a superconducting coil 2 is contained in an adiabatic vessel so as to cause a superconducting transition of the superconducting coil 2, and then, liquid helium 4 is injected thereinto, thereby causing the superconducting transition.
No matter how excellent the adiabatic vessel is, the liquid helium 4, which is very expensive, evaporates owing to penetrating heat conducted and radiated from the outside and to penetrating heat transferred from a current lead. Consequently, an amount of evaporated liquid helium, thus, the running cost of the superconducting equipment depends largely upon the quality of the current lead 3. This is very serious problem in putting this equipment to practical use.
Further, the following two kinds of heat are considered as such penetrating heat coming from a current lead:
(A) Penetrating heat produced due to the thermal conduction from the outside of the equipment.
(B) Joule heat generated from a current lead by actually energizing a superconducting coil with a large current.
Although Joule heat herein-above described as of the kind (B) can be reduced by increasing the area of a (transverse) section of the current lead and decreasing the length thereof, the penetrating heat is increased in such a case by an amount of heat transferred owing to the thermal conduction herein-above described as of the kind (A).
Thus, the conventional superconducting equipment is designed so that the sum of amounts of heat of the aforementioned kinds (A) and (B) is minimized. However, it is inevitable that a certain amount of (penetrating) heat penetrates into the adiabatic vessel of the superconducting equipment. Consequently, the conventional superconducting equipment has not come to change a situation in which most of penetrating heat is originated from a current lead itself.
To overcome such a situation, there has been provided a current lead that uses an oxide high-temperature superconductor. Generally, the critical temperature of a high-temperature oxide superconductor is very high, so that the superconducting transition thereof can be easily achieved even in the case of using inexpensive liquid nitrogen (at 77 K). When an oxide high-temperature superconductor is used in liquid nitrogen as a material of a current lead, heat of the aforementioned kind (B), namely, Joule heat is not generated because the current lead is in a superconducting state. Further, the thermal conductivity of the oxide high-temperature superconductor is very low as compared with metallic copper, because of the fact that this oxide high-temperature superconductor is made of ceramics. Thus, an amount of penetrating heat of the kind (A) can be considerably reduced. Consequently, a total amount of penetrating heat can be largely decreased in comparison with a metallic-copper current lead.
However, the connection between such a current lead and the superconducting coil is thus established between dissimilar materials, namely, metal and ceramics. Therefore, when the current lead is connected with the superconducting coil immersed in liquid helium, large contact resistance occurs. As a result, Joule heat is generated in the connecting portion therebetween. Consequently, an amount of evaporated liquid helium increases.
To prevent such a problem from coming up, electrode portions are first made by plating both end parts of each rod-like (or stick-like) current lead with silver foil or by covering such end parts thereof with silver paste. Then, the plated or covered end parts thereof are baked or burnt so as to form metallic electrode structures. Thereby, the connection between similar materials, namely, between metals is realized. Thus, an attempt has been made to the aforementioned contact resistance. Consequently, the contact resistivity of the electrode portions (hereunder sometimes referred to as "electrode-portion contact resistivity") is reduced to about 1 .mu..OMEGA..multidot.cm.sup.2.
As above described, a reduction in the electrode-portion contact resistivity to about 1 .mu..OMEGA..multidot.cm.sup.2 has been achieved by forming metallic electrode portions at both end parts of each current lead constituted by an oxide high-temperature superconductor. Even in such a case, when energizing the superconducting coil with an electric current of, for example, 1000 A, a calorific value W is estimated as follows by assuming that the area of a contact surface portion is, for instance, 1 cm.sup.2, for brevity of description: EQU W=R.multidot.I.sup.2 =(1 .mu..OMEGA.).multidot.(1000 A).sup.2 =1 J/s.
Thus, the generated Joule heat is still high. If Joule heat generated during energizing the superconducting coil is high, an amount of evaporated coolant such as liquid helium increases. Consequently, the conventional current lead has a problem in lack of economical practicality.
Further, it is confirmed that the critical current density of the oxide superconductor itself composing the current lead changes under the influence of a heat treatment step to be performed for the purpose of forming metallic electrode portions. Thus, there is the possibility of an occurrence of a large reduction in the critical current density according to conditions for the heat treatment. When the critical current density of the oxide superconductor itself lowers on certain conditions, a single current lead cannot be suited to (or adapted for) use in ordinary superconducting equipment that needs to be energized with a large current. Moreover, in the case of using a plurality of current leads connected in parallel with one another, the number of current leads becomes large, so that the equipment has problems in increase in weight and size thereof.
Although studies on a problem of still more reducing the electrode-portion contact resistivity have been conducted, such a problem is not solved yet. Further, attempts have been made to study and design optimum heat treatment conditions by which the critical current density of the oxide superconductor is not reduced. However, such optimum conditions are not detected yet.
The present invention is accomplished to solve the aforementioned problems of the prior art.